In /etc/syslog.conf add the following lines to log notice alerts to /var/log/samba-audit.log

local7.notice /var/log/samba-audit.log

Add an entry in newsyslog.conf to rotate the logs:

/var/log/samba-audit.log 644 100 1000 * JC

Where:
644: is the permissions of the file
100: is the amount of logs to keep
1000: is the max file size (1MB)
*: is rotation period (blank if going by size alone)
JC: C is compression J is create new file if doesn't already exist

Restart Samba

/usr/local/etc/rc.d/samba restart

A reload wont do it you'll have to restart the entire daemon. You also might have to restart syslogd